>>34246320I think that UB are just "pokemon gifted by the power of light" (Z power). Guzzlord's world reveals each UB world is actually our world in a (very) different timeline, so let's say the UBs are just pokemon that exist exclusively in other timelines (unlike others like Altaria, Regirock, etc that appear in parallel worlds and our world) AND are gifted by this light, resultating in the Ultraboost ability just like our world's pokemon can be gifted (in a smaller degree) with the light if they live in an ecosystem infused with this light energy (which are the totem pokemon and the trial sites, according to the URS, just places full charged with light so crystalizations of the light, the Z crystals and big and stronger pokemon with auras are common). Cosmog line and Necrozma are different in the way they generate the light instead being affected with it by other pokemon's influence.
So at the end every pokemon species which is powered with the "light" and can't be found in our main timeline is an UB. Maybe the "from different timeline" thing isn't even necessary and change into a more powerful pokemon due to "light" overexposure is enough to use the term and someday the totem pokemon would evolve (in the darwinian sense of the word) into "UBs" too
